A good trainer can be invaluable, especially if you're new to dogs. This person can help you understand and deal with the progression from puppy to adolescent to adult. You'll learn how to teach your Yorkie good manners and perhaps some useful tricks, and see if you might want to continue into competition.
But trainers are not all alike, so you have to invest some time and thought into choosing the one for you and your dog. While books can teach you a lot, you can't ask them questions and they can't observe your dog. You should certainly still read books but also take at least one class.